Known in the art is an internal combustion engine having arranged in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOx produced when burning fuel under a lean air-fuel ratio is stored in the NOx storing catalyst. On the other hand, when the NOx storing capability of the NOx storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby the NOx is released from the NOx storing catalyst and reduced.
However, the fuel and lubrication oil contain sulfur. Therefore, the exhaust gas contains SOx. This SOx is stored in the NOx storing catalyst along with the NOx. However, this SOx is not released from the NOx storing catalyst just by making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOx stored in the NOx storing catalyst gradually increases. As a result, the amount of NOx which can be stored ends up gradually decreasing.
Known in the art therefore is an internal combustion engine having an SOx trap agent arranged in the engine exhaust passage upstream of the NOx storing catalyst so as to prevent SOx from being sent to the NOx storing catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 2004-92524). In this internal combustion engine, the SOx contained in the exhaust gas is trapped by the SOx trap agent. Therefore, the SOx is prevented from flowing into the NOx storing catalyst. As a result, the storing capability of the NOx can be prevented from dropping due to storage of SOx.
However, in this internal combustion engine, before the SOx trap agent becomes saturated in SOx trap ability, SOx is released from the SOx trap agent. In this case, to make the SOx trap agent release the SOx well, when the temperature of the SOx trap agent is the SOx release temperature, the exhaust gas has to be made a rich air-fuel ratio. Therefore, in this internal combustion engine, to make the SOx trap agent release the SOx, when the temperature of the SOx trap agent is the SOx release temperature, the exhaust gas is made a rich air-fuel ratio.
Further, this internal combustion engine is provided with a bypass exhaust passage bypassing the NOx storing catalyst so as to prevent the SOx released from the SOx trap agent from flowing into the NOx storing catalyst. When SOx is released from the SOx trap agent, the exhaust gas flowing out from the SOx trap agent is guided to the inside of the bypass exhaust passage.
On the other hand, in this internal combustion engine, when the SOx trap amount of the SOx trap agent becomes a certain amount or more, even if the temperature of the SOx trap agent is less than the SOx release temperature when the exhaust gas is made a rich air-fuel ratio so as to make the NOx storing catalyst release NOx, the SOx trap agent ends up releasing SOx and therefore the SOx ends up being stored in the NOx storing catalyst. Therefore, in this internal combustion engine, when the SOx trap amount of the SOx trap agent becomes a certain amount or more, the exhaust gas is prohibited from being made a rich air-fuel ratio.
However, in this case, if using an SOx trap agent which does not release SOx when the exhaust gas is made a rich air-fuel ratio to make the NOx storing catalyst release the NOx, SOx is no longer stored in the NOx storing catalyst. Further, when NOx should be released from the NOx storing catalyst should release NOx, it is possible to make the exhaust gas a rich air-fuel ratio at all times. However, so long as giving the SOx trap agent the function of releasing SOx such as in this internal combustion engine, it is difficult to prevent SOx from being released when the exhaust gas becomes a rich air-fuel ratio.